This invention relates to a pumping up hydroelectric power plant utilizing a reversible pump/turbine and a booster pump operable in series when the pump/turbine is operated in the pumping mode.
Recently, pumping up hydroelectric power plants operable under high heads are being constructed widely. However, when the pump/turbine provided therein is of a single speed type, the head under which the pump/turbine is operated at the maximum efficiency in the pumping mode is considerably lower than that in the turbine mode of operation also at the maximum efficiency, and therefore it is found difficult to operate the pump/turbine in both of the two modes at their highest efficiencies under an operating head condition assigned to the pumping up power plant.
In order to overcome this difficulty, there has been proposed and constructed a pumping up hydroelectric power plant wherein a booster pump is further provided in a by-pass portion of the suction side tunnel of the pump/turbine in such a manner that the booster pump is operated in series with the pump/turbine with a portion of the head available at that time assigned to the booster pump during the pumping operation of the pump/turbine.
In such an arrangement of the pumping up hydroelectric power plant, during the pumping operation since the total head of the power plant is shared between the pump/turbine and the booster pump, the actual head under which the pump/turbine is operated in the pumping mode can be reduced by an amount assigned to the booster pump, and therefore the pump/turbine can be operated in both pumping mode and turbine mode at the points of highest efficiencies even under the operating head condition assigned to the power plant, thus improving the total efficiency of the power plant.
However, in the above described construction of the power plant, since the main pump/turbine and the booster pump, inherently having different hydraulic characteristics, are operated in series along a single water passage (or suction side tunnel), the two machines tend to interfere with each other in their transient conditions, thus rendering the operations to be utterly unstable.
For instance, when the pump/turbine and the booster pump having different deceleration characteristics are simultaneously interrupted from the power system, a difference in flow-rate due to the different deceleration characteristics tends to cause severe variation in water pressure in the draft tube.
More specifically, where the deceleration of the booster pump is larger than that of the main pump/turbine, the rotating speed of the latter becomes higher than that of the booster pump after their interruption from the power system, and a reduction in speed of the booster pump is prevented by the water flowing through the main pump/turbine. The operation of the booster pump is thus deviated from the designed condition, tending to cause a separation of water layers (or cavitation) which in turn tends to damage the booster pump and other parts related thereto.
On the other hand, when the deceleration of the main pump/turbine is greater than that of the booster pump, the quantity of water pumped up by the main pump/turbine per unit time is rapidly reduced. However, since the rotating speed of the booster pump is still higher than that of the main pump/turbine, an extremely high pressure is inevitably induced at the delivery side of the booster pump, with a result that the booster pump and other related parts are thereby damaged.